So-called "prepayment" cards are known that may be purchased from an organization which supplies a service, with the card including magnetic or electronic memory means for storing a credit balance. With such cards, each time a service is supplied, the balance is reduced by an amount which corresponds to the service supplied. When the entire initial credit has been used, the card is thrown away. This system is well-known for pay phones and for gasoline stations. The pay phone or the gas pump is provided with a "card reader" which identifies the card and which deducts an amount therefrom corresponding to the telephone call made or to the quantity of gas delivered.
The drawback of such cards is that the cost of manufacturing them is not negligible compared with the necessarily small amount of money which a user is prepared to "prepay" in order to obtain the service provided.
In order to remedy this drawback, so-called "re-fillable" or revalidatable cards have been proposed, i.e. cards in which it is possible to record a new balance after the initial balance has been completely spent by paying the amount of credit by which the card is refilled. The problem of this type of card naturally consists in preventing the cardholder from fraudulently refilling the card without making the corresponding payment to the organization which provides the service. To do this, proposals have been made to split the memory in which the amounts of credit are recorded into a plurality of zones that should be validated in succession. In order to verify that successive validations are licit, the card includes a zone in its memory for validating the successively validatable credit zones. Access can be obtained to the validation zone only by means of a secret encoding key which is stored in the card.
The defects of such a system are clear. Firstly it is inapplicable to a card having a magnetic memory. Secondly, even with a card having an electronic memory, it requires access-controlling circuits which are complex and therefore expensive in addition to the memory.
In order to remedy these drawbacks, an object of the invention is to provide a system for supplying services using portable data mediums, which system is compatible with magnetically recording successive balances and which allows the data medium to be successively refilled under acceptable conditions of security.